Moisture regulation 183 surface area)and the length of time in storage are all known, the moisture ingress can be determined. This value represents the amount removed by the desiccant to protect against possible condensation within the package or damage due to prolonged exposure in a high humidity environment The permeability coefficient is a material constant which specifies the olume of gas which will pass through a test material of known surface area and thickness in a fixed time, with a given partial pressure difference. The coefficient varies with temperature. Permeability coefficients for packaging materials can usually be obtained from the material supplier or, more appropriately, permeability data on the actual package may be available Published material permeability values can be found not only for the packaging material, but also for desiccant packaging(a sachet, for example) as well Permeability coefficients can be converted into permeability rates defined by the origin of the driving force involved(whether it be water concentration, partial pressure or mole fraction). Water vapour partial pressure data are acquired through thermodynamic steam tables. These rates may have to be adjusted to reflect actual shelf-life conditions. Moisture ingress(mg water/day, for example)across a package wall for given relative humidity conditions inside the package is determined by the use of a general material balance for water, namely Flow of H2O=ApkgPpkg(Cout-Cin) where Akg =surface area of package Ppkg= permeability rate of package material Cout concentration of water Cin= concentration of water in headspace The permeability rate of the package material needs to be adjusted both for the thickness of the material and the partial pressure differential. It is important to note that the reaction rates are assumed to be relatively rapid compared to the rate of diffusion, so that the process is diffusion controlled. The amount of water permeating into a package during shelf-life depends largely on the type of packaging(e.g, flexible, rigid) and the packaging materials that are used. For example, various flexible packaging materials exhibit different MVTRs. Water ingress into flexible packaging can only be roughly estimated from such data This is because the contribution of moisture ingress from the seal is not included in material permeability data. Desiccants, which address moisture concerns, are usually chosen by directly running tests under the intended application However, more rigorous testing can become time-consuming and expensive 9.7.2 Desiccant selection process In order to maximise success in the selection of a desiccant, the properties of both the commercially available desiccant and the package material(s) must besurface area) and the length of time in storage are all known, the moisture ingress can be determined. This value represents the amount removed by the desiccant to protect against possible condensation within the package or damage due to prolonged exposure in a high humidity environment. The permeability coefficient is a material constant which specifies the volume of gas which will pass through a test material of known surface area and thickness in a fixed time, with a given partial pressure difference. The coefficient varies with temperature. Permeability coefficients for packaging materials can usually be obtained from the material supplier or, more appropriately, permeability data on the actual package may be available. Published material permeability values can be found not only for the packaging material, but also for desiccant packaging (a sachet, for example) as well. Permeability coefficients can be converted into permeability rates defined by the origin of the driving force involved (whether it be water concentration, partial pressure or mole fraction). Water vapour partial pressure data are acquired through thermodynamic steam tables. These rates may have to be adjusted to reflect actual shelf-life conditions. Moisture ingress (mg water/day, for example) across a package wall for given relative humidity conditions inside the package is determined by the use of a general material balance for water, namely Flow of H2O ˆ ApkgPpkg…Cout ÿ Cin† where Apkg ˆ surface area of package Ppkg ˆ permeability rate of package material Cout ˆ concentration of water Cin ˆ concentration of water in headspace. The permeability rate of the package material needs to be adjusted both for the thickness of the material and the partial pressure differential. It is important to note that the reaction rates are assumed to be relatively rapid compared to the rate of diffusion, so that the process is diffusion controlled. The amount of water permeating into a package during shelf-life depends largely on the type of packaging (e.g., flexible, rigid) and the packaging materials that are used. For example, various flexible packaging materials exhibit different MVTRs. Water ingress into flexible packaging can only be roughly estimated from such data. This is because the contribution of moisture ingress from the seal is not included in material permeability data. Desiccants, which address moisture concerns, are usually chosen by directly running tests under the intended application. However, more rigorous testing can become time-consuming and expensive. 9.7.2 Desiccant selection process In order to maximise success in the selection of a desiccant, the properties of both the commercially available desiccant and the package material(s) must be considered. The conditions involving optimum product preservation and performance are analysed experimentally and/or by seeking published literature values. Moisture regulation 183